1. Field of the Invention
This invention relates in general to apparatus and a method for monitoring a system operation and more particularly to such apparatus and method for monitoring and visually displaying an axial power distribution within the core of a nuclear reactor that employs parameters that are monitored exterior to the reactor core.
2. Description of the Prior Art
The controlled release of large amounts of energy through nuclear fission is now quite well known. In general, a fissionable atom such as U.sup.233, U.sup.235, or PU.sup.239 absorbs a neutron in its nucleus and undergoes a nuclear disintegration. This produces on the average, two fission products of lower atomic weight and great kinetic energy, several fission neutrons, also of high energy, and fission gamma rays.
The kinetic energy of the fission products is quickly dissipated as heat in the nuclear fuel. If, after this heat generation, there is at least one neutron remaining which induces a subsequent fission, the fission reaction becomes self-sustaining and the heat generation is continuous. The heat is removed by passing a coolant through heat exchange relationship with the fuel. The reaction may be continued as long as sufficient fissionable material exists in the fuel to override the effects of fission products and other neutron absorbers which may also be present and neutron leakage.
In order to maintain such fission reactions at a rate sufficient to generate useful quantities of thermal energy, nuclear reactors are presently being designed, constructed and operated in which the fissionable material or nuclear fuel is contained in fuel elements which may have various shapes, such as plates, tubes or rods. These fuel elements are usually provided on their external surfaces with a corrosion resistant, non-reactive cladding which contains no fissionable material. The fuel elements are grouped together at fixed distances from each other in a coolant flow channel or region as a fuel assembly, and a sufficient number of fuel assemblies are arranged in a spaced array to form the nuclear reactor core capable of the self-sustained fission reaction referred to above. The core is usually enclosed within a reactor vessel.
Commonly, in pressurized water reactors, a neutron absorbing element is included within the cooling medium (which also functions as a moderator) in controlled variable concentrations to modify the reactivity, and thus the rate of heat generation within the core, when required. In addition, control rods are interspersed among the fuel assemblies, longitudinally movable axially within the core, to control the core's reactivity and thus its power output. There are three types of control rods that are or have been employed for various purposes. Full length rods, which extend in length to at least the axial height of the core, are normally employed for reactivity control and, in a number of current applications, for axial power distribution control as well. In other current applications, part length control rods, which have an axial length substantially less than the height of the core, are specifically used for axial power distribution control. In addition, reactor shut-down rods are provided for ceasing the sustained fissionable reaction within the core and shutting down the reactor. The part length rods, when used, and the full length rods are arranged to be incrementally movable into and out of the core to obtain the degree of control desired.
As a by-product of the fissionable reaction, both through a process of beta decay of radioactive iodine-135 and, by direct yield from the fission reaction, xenon-135 is created. Xenon-135 has the property of having a uniquely large neutron cross section and, therefore, has a significant effect on the power distribution within the core and on overall core reactivity. While other forms of reactivity management are directly responsive to control, the xenon concentration within the core creates serious problems in reactor control in that it gives rise to an unavoidable short term positive feedback mechanism. It exhibits a relatively long decay period and requires up to approximately 40 hours after a power change to reach a steady state value.
Short and long term transients in the core axial power distribution have created problems throughout the history of reactor operation for several reasons. Normally, coolant flow through the fuel assemblies is directed from a lower portion of the core to the upper core regions, resulting in a temperature gradient axially along the core. Changes in the rate of the fission reaction, which is temperature dependent, will thus vary the axial power distribution. Normal design practice insures that axial moderator temperature variations are always characterized by negative local and core average feedback effects. Secondly, the axial variation in the power distribution varies the xenon axial distribution which further accentuates the variations in power axially along the core. This can lead to a xenon induced axial power distribution oscillation which can be unstable and which requires continuous corrective operator intervention. Thirdly, insertion of control rods from the top of the core, without proper consideration of the past operating history of the reactor, can worsen the axial power peaking. Unlimited axial power peaking or excessive rates of change of power peaking will result in fuel failure and release of radioactive material into the coolant.
The series of changes in reactor core power output in a nuclear electrical generating plant that must be made in order to accommodate the daily changes in electrical demand that commonly occur in a typical electrical utility grid is generally referred to as load follow. One load follow control program currently recommended by reactor vendors utilizes the movement of the full length control rods for power level increases and decreases and the part length control rods to control xenon induced spatial axial power oscillations and to shape the axial power profile. Typically, changes in reactivity associated with changes in the xenon concentration are generally compensated for by opposing changes in the concentration of the neutron absorbing element within the core coolant or moderator. Some vendors have identified and recommended load follow programs or strategies that utilize appropriate timed movement of the full length control rods. This is done to compensate for the effects of power level changes on overall core reactivity and, at least in part, to prevent the establishment of unacceptable axial power distributions caused by spatial xenon-135 transients induced by power level changes and control rod movement. In these strategies, part length control rods are not needed for axial power distribution control and, as a result, they are commonly not installed in the reactors manufactured by those vendors. In either mode of operation, the control rods, either part length or full length, are moved to maintain a parameter called the axial offset within some prespecified band, typically within the range of + or -15%.
The axial offset is a useful parameter for characterizing the axial power distribution and is defined as: EQU AO=(P.sub.t -P.sub.b)/(P.sub.t +P.sub.b) (1)
where P.sub.t and P.sub.b denote the fraction of power generated in the top half and bottom half of the core, respectively.
The concept of axial offset derives from the earliest attempts to synthesize, on the basis of readily measurable quantities, the core average axial power distribution in an operating nuclear power reactor. In these early efforts, the core average axial power distribution was commonly approximated by a Fourier series of sine functions bounded by the extrapolated axial length of the core: ##EQU1## where q(z) is a measure of core average axial power level, in units of kw/ft for example;
Z represents the extrapolated core height; and PA1 z represents axial elevation above the extrapolated lower core limit. PA1 - lack of an on-line, real-time knowledge of core axial power distribution which leads to uncertainty in determining the margin to various core limits. The uncertainty directly affects the operating margin for the plant in performing load follow and other maneuvers. PA1 - lack of a detailed on-line, real-time core axial power distribution calculation which precludes the ability to determine axial xenon and iodine distributions which are helpful in inferring the future behavior of the core. Such an inference is valuable in operating various systems, for example boron systems, in an anticipatory manner to avoid potentially undesirable power distributions.
It was early recognized, granted a few acceptable simplifying assumptions, that the ratio A.sub.2 /A.sub.1 appearing in eqn. (3) could be correlated directly with the axial offset parameter defined by eqn. (1). Thus, the assupton was made, early on, that effective first order control of the core average axial power distribution could be established and maintained by adjusting either full length or part length control rod position in the core, as necessary, to hold the axial offset parameter value within prespecified limits. In actual practice, since the values of P.sub.t and P.sub.b in eqn. (1) can not conveniently be observed directly, an alternative measurable parameter, commonly called the ex-core axial offset, is defined as: EQU AO.sub.ex-core =(I.sub.t -I.sub.b)/(I.sub.t +I.sub.b) (4)
where I.sub.t and I.sub.b are the compensated electrical currents generated, respectively, by the top and bottom ex-core neutron detectors of a two-section excore detector system. It was early noted, and consistently observed, that the true axial offset in the reactor core, as defined by eqn. (1), was reliably related to the readily measurable ex-core axial offset of eqn. (4) by the following: EQU AO=a+b*AO.sub.ex-core ( 5)
By repeated analysis of experimental data from a wide spectrum of operating nuclear power reactors manufactured by the present assignee, the value of the parameter a has been found to always be close to 0.0 and the value of the parameter b has been found to fall in the range 1.3 to 1.8.
However, the currents produced by the top and bottom ex-core detectors of conventional two-section ex-core long ion chamber detector systems provide sufficient information to permit evaluation of only the first two coefficients A.sub.l and A.sub.2 of the expansion in eqn. (1), thereby limiting the expansion to the first two terms.
More recently however, it has been shown in the art, that in practical situations, relatively large uncertainties must be associated with estimates of the location and amplitude of peak values in axial power profiles constructed on the basis of the responses of the detectors in typical two-section "long ion chamber" ex-core neutron detector configurations. For one thing, axial power pinching, which results in a large axially centered power peak, can occur with a low or zero axial offset. The mere prospect that such axially centered power peaks could occur and could pass undetected by the existing power distribution monitoring system could result in a reactor power penalty. Under current licensing criteria, this would require that the reactor be operated at a reduced power level in order that such potential peaks could not exceed conservative specified magnitudes. Alternatively, axial power flattening, which results in an axially centered depression in the axial power profile and in the appearance of two axially symmetric regions of unusually high power density near the top and bottom of the reactor core, can also occur. This, too, could pass undetected if a conventional two section ex-core neutron detector system were the only monitoring system in use. This potential presence of a region of unusually high power density near the top of the core could cause significant concerns regarding the possibility of the undetected establishment of a departure from nucleate boiling condition in that region. Here again, the mere prospect that such a situation could arise, could again result, under current licensing criteria, in a derating of the allowable power level at which an affected nuclear power reactor is permitted to operate to insure that the projected condition could not arise.
As the significance of these somewhat more subtle characteristics of core average axial power distribution became apparent, two basic approaches to axial power distribution monitoring and control emerged. One approach followed the route of developing enhanced monitoring hardware. Initially, such hardware included ex-core neutron detector arrangements in which more than two neutron detectors were combined in a single grouping, wherein each detector "sees" a different axial region of the peripheral region of the reactor core immediately adjacent to the location of the ex-core detectors. Subsequently, such hardware included fixed in-core neutron or gamma ray detectors that are installed within the active region of the core. These detectors provide direct responses characteristic either of the local neutron flux level or of the local gamma ray flux level. As is well known, as the number of detectors monitoring the nuclear processes in successive axial regions of a radially common portion of a reactor core is increased, so does the precision with which the axial power distribution can be synthesized. In simple terms, for each additional axially independent detector added to the hardware arrangement of an overall axial detector configuration, one additional term can be added to the Fourier sine expansion of eqn. (2) or (3) and the resulting precision in axial power distribution synthesis improves thereby.
The alternative approach that emerged in response to the realization that higher order components of the core average axial power distribution could have significant effects on reactor core operability and ultimately on reactor safety was one of retaining the basically simple and rugged two-section ex-core neutron detector hardware, the so-called "long ion chamber" arrangement, and evolving operating strategies that guaranteed, by prior analysis, that no unacceptably adverse axial power distributions could be established if the constraints of the specified operating strategies were faithfully observed. As a consequence of this alternative approach, a significant number of currently operating nuclear power reactors in various nations in the world, including to a large degree, those operating nuclear power reactors in the United States, have installed only two-section ex-core neutron detector systems or arrangements and have a small possibility, given current nuclear power reactor licensing policy and constraints, of utilizing ex-core neutron detector arrangements consisting of more than two sections of axially independent neutron detectors. The alternative option of installing more multisection neutron detector arrangements exists, but it is not considered commercially viable by the operators of the affected nuclear power plants. It is to be noted that in all operating nuclear power plant installations in which only two-section neutron detector arrangements are provided, there are also provided the means for periodically obtaining very precise measurements of the actual three dimensional core power distribution. This is accomplished by utilizing an installed movable in-core detector system. These installations are also routinely supplied with a core-exit thermocouple system which allows the continuous, on-line monitoring of the coolant temperature at the exit of about one fourth of the fuel assemblies and provides, therefore, the ability to infer values of coolant enthalpy rise (i.e. total local thermal power) at each monitored location. Hitherto, little use has been made of the information available from the core-exit thermocouple system.
Some drawbacks of the alternative approach of using only the movable in-core system and the two-section ex-core detector system arrangement for power distribution monitoring include:
To establish an effective load follow capability, a substantially constant axial power profile will have to be maintained or the axial power profile will have to be continuously managed throughout load follow operation. However, to effectively maintain a substantially constant axial flux profile or to continuously manage the axial flux profile, a monitoring system is required that has the capability of substantially reconstructing the axial flux pattern within the core so that variations therein can be accurately compensated for before a xenon maldistribution is effected.
If one more term, A.sub.3 sin(3*.pi.*z/Z), representing the so-called "second overtone" of the power distribution, is included in the expansion of eqn. (`), the uncertainties are reduced significantly and the resulting estimate of power shape is usable for power peaking and other analysis. The first three terms of the expansion carry most of the information regarding the effects of current operating conditions, power level and control rod position in particular as well as some of the effects of burnup on axial power distribution. Addition of a fourth term of the form A.sub.4 sin(4*.pi.*z/Z), which term is relatively insensitive to either short term or long term operating conditions, provides relatively little further improvement in the power shape definition. The fifth term embodies most of the long term, i.e., burnup dependent, effects on axial power distribution and so its coefficient changes quite slowly as the axial burnup distribution in the core changes.
Thus, a need exists for a method and apparatus for monitoring the information available from the ex-core detectors, as well as information available from other ex-core detectors, to produce an estimate of the core axial power distribution over the entire height of the core which is suitably accurate for use as a primary diagnostic tool in operating a nuclear reactor.